1. Field of the Invention
The present invention relates to a mobile communication terminal and a mobile communication control method, and particularly, to a mobile communication terminal and a mobile communication control method for use in a mobile communication system.
2. Description of the Related Art
A mobile communication terminal such as a cellular phone device executes a PLMN (Public Land Mobile Network) selection to search for a cell available in a mobile communication network when the power is turned on or when the terminal is out of the range of a cell. A description will be given of an operation for the PLMN selection executed by a mobile communication terminal.
An operation of searching for a mobile communication network is mainly categorized into the following three stages.
In the first stage, the mobile communication terminal detects the presence of a mobile communication network by measuring a reception level or the like of a radio signal transmitted with a certain power from a base transceiver station of the mobile communication network for an RAT (Radio Access Technology) supported by the mobile communication terminal or the frequency.
In the second stage, the mobile communication terminal receives notice information broadcasted from the mobile communication network detected in the first stage to acquire a Mobile Country Code (MCC) included in the notice information and a Mobile Network Code (MNC) indicating a mobile communication provider, thereby recognizing the type of mobile communication network to select the PLMN with the highest priority.
In the third stage, after selecting the mobile communication network with the highest priority, the mobile communication terminal further acquires, from the notice information, other information, such as adjacent cell information, which is necessary for receiving services. The mobile communication terminal performs a location registration with respect to the cell, if necessary. The mobile communication terminal is capable of receiving services from the mobile communication network when the mobile communication network approves a location registration request from the mobile communication terminal.
Frequency support is optional in the 3GPP (3rd Generation Partnership Project) specifications. Therefore, communication services for a specific frequency are not provided by a communication provider that does not support the frequency, or in an area where the use of a certain frequency is not approved even if the communication provider supports the frequency. Furthermore, in some areas, another communication provider operates at the frequency identical to that of a certain area.
According to the 3GPP specifications at present, cancellation is performed in the unit of frequency, if location registration fails for a specific reason in the network operation. Such a cancellation is called frequency cancellation or making the frequency unavailable. The frequency cancellation is, for example, the removal of the frequency or for a certain period of time from targets of the PLMN selection or cell selection so that an unnecessary PLMN selection or cell selection is not performed in the frequency when the location registration failed, because the location registration was performed for a non-contract provider. That is, a frequency cancellation process of ignoring a specific frequency in the radio waves included in a cell is executed during the certain period of time. This certain period of time is referred to as frequency cancellation time. This period of time is common to all frequencies, in other words, the same period of time is set.
The cell selection is an action of searching for and waiting for a cell of the selected PLMN, while the mobile communication terminal is moving from the range of a cell. The PLMN selection operation starts if a cell cannot be found in the cell selection.
A technique for selecting an optimal base transceiver station based on a cell selection table is described in Japanese Patent Application No. 2002-27522.
No problem will occur, if individual frequencies are assigned to the providers for providing communication services to mobile communication terminals. That is to say, no problem will occur, if the frequencies are assigned to adjacent cells in a unified manner, in the unit of frequency. However, a problem will occur, if providers are different in adjacent cells with respect to the identical frequency.
For example, after the mobile communication terminal has transmitted the location registration request to the network, the fact that the cell cannot be connected is recognized when the location registration request is rejected. Therefore, an unnecessary location registration request is transmitted, so the battery as a power source of the mobile communication terminal is consumed, and a radio resource is also consumed for the unnecessary request. Subsequently, the frequency is rejected for a certain frequency cancellation time. In this case, since the frequency cancellation time is set to a common value to all frequencies in a conventional mobile communication terminal, the efficiency of the frequency cancellation is poor when the mobile communication terminal moves around areas where the frequencies are not assigned to providers in a unified manner. That is, in a case where the frequency cancellation time is set long, there is a problem in that the frequency is rejected when the mobile communication terminal returns the area of a cell with the frequency of a contract provider even if the connection is available. These problems will be described with reference to FIGS. 10 and 11.
FIG. 10 shows an example of a frequency cancellation process when the frequencies are assigned to the communication providers in a unified manner (that is, frequencies are assigned to communication providers in all cells in a unified manner).
The cells represented by “contract” are cells managed by a communication provider with which the user has made a contract (hereinafter, referred to as “contract cell”). Meanwhile, the cells represented by “another” are cells managed by communication providers (that is, another communication provider, hereinafter, referred to as “non-contract cell”). In this example, identical frequencies are assigned to the communication providers in all cells. Specifically, the frequencies are assigned as follows.
In an area A, cells sc11 and sc12 are contract cells for frequencies f1 and f2. On the other hand, the cells sc11 and sc12 are non-contract cells for frequencies f3 and f4.
In addition, in an area B, cells sc21 and sc22 are contract cells for the frequencies f1 and f2. On the other hand, the cells sc21 and sc22 are non-contract cells for the frequencies f3 and f4.
Further, in an area C, cells sc31 and sc32 are contract cells for the frequencies f1 and f2. On the other hand, the cells sc31 and sc32 are non-contract cells for the frequencies f3 and f4.
The problems will not occur, if individual frequencies are assigned to the providers, respectively, and the frequencies are allocated to adjacent cells in a unified manner in the unit of frequency as shown in FIG. 10. This is because the location registration is not requested to another non-contract provider, even if the mobile communication terminal is located in the cell sc12 at the frequency f2 in the area A, then moves from the area A and passes through the area B, and finally reaches the cell sc31 for the frequency f2 in the area C.
Meanwhile, FIG. 11 shows an example of another frequency cancellation process when the frequencies are not assigned to the communication providers in a unified manner (that is, a specific frequency is assigned to different communication providers in areas). In this example, a specific frequency is assigned to different communication providers in some areas. Specifically, the frequencies are assigned as follows.
In the area A, the cells sc11 and sc12 are contract cells for the frequencies f1, f2, and f3. On the other hand, the cells sc11 and sc12 are non-contract cells for the frequency f4.
In the area B, the cells sc21 and sc22 are contract cells for the frequencies f1 and f2. On the other hand, the cells sc21 and sc22 are non-contract cells for the frequencies f3 and f4.
In the area C, the cells sc31 and sc32 are contract cells for the frequencies f1, f2, and f3. On the other hand, the cells sc31 and sc32 are non-contract cells for the frequency f4.
As shown in FIG. 11, in a case where the frequencies are assigned in a unified manner, the frequency cancellation process initiates in the area B if the mobile communication terminal is located in the cell sc12 at the frequency f3 in the area A, then moves from the area A and passes through the area B, and finally reaches the area C. The frequency cancellation process is continuously performed in some cases, after the movement to the area C. This causes a problem that the frequency is rejected in the area C even though it is connectable at the frequency f3.
Japanese Patent Application No. 2002-27522 merely describes a technique for selecting cells with large ranges and small ranges and cannot solve the problems.